x86/x2apic: Mark set_x2apic_phys_mode() as __init

x86/x2apic: Mark set_x2apic_phys_mode() as __init

set_x2apic_phys_mode() is only called as part of early_param()
initialization - so mark it as __init.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180117034543.26723-1-douly.fnst@cn.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>